Binder-free silver halide photographic process



March 26, 1968 I. E.-THEODOROU 3,375,112

BINDER-FREE SILVER HALIDE PHOTOGRAPHIC PROCESS Filed 001;. 28, 1963 INVENTOR.- IGNATIUS E 4T HEODOROU BY M A. smma" m m- HM/wuz W1 041M 4% 5m ATTORNEYS Patented Mar. 26, 1968 3,375,112 BINDER-FREE SILVER HALIDE PHOTOGRAPHIC PROCESS Ignatius E. Theodorou, Watertown, Mass., asslgnor, by

mesne assignments, to Technical Operations, Incorporated, a corporation of Delaware Filed Oct. 28, 1963, Ser. No. 319,452 4 Claims. (CI. 9667) This invention relates to photography and more particula-rly to novel photographic media and processes for using such media to provide visible images. The photographic media of the present invention are of the type comprising a discrete layer or stratum of photosensitive, contiguous, microcrystals of silver halide. Media of this type and a process for manufacturing same, as by a vacuum evaporation technique, are disclosed in French Patent No. 1,267,623 granted June 21, 1961 to Technical Operations, Incorporated. Media of this evaporated-layer type can readily be distinguished from other silver halide photographic media, particularly emulsion-type photographic films, not only structurally and by method of manufacture, but for some purposes by the superior acuteness, high resolution and quick developability of the binder-free stratum.

Because the evaporated-layer type of media are formed of microcrystalline silver halide substantially without a protective interstitial binder, such as the gelatin in silver halide emulsions, they are somewhat more susceptible than emulsion-type film to damage from mechanical abrasion or chemical action from airborne pollutants such as hydrogen sulfide or the like. Consequently, it has been suggested that protective coatings be applied to the stratum prior to exposure and development, thereby offering protection to the stratum during storage. It has also been suggested that, after exposure and development, a coating can be applied to provide protection to the finished silver image. Coatings or films applied prior to processing preferably are those which are either permeable to or soluble in the developer vehicle. The use of soluble films of course results in the destruction of the protective film during processing. On the other hand, the need to apply coatings or films after processing to protect the silver image is considered burdensome in the use of the photographic medium and the very act of application of the protective coating can often cause damage to an image by abrasion.

A principal object of the present invention is to provide a novel photographic medium which includes a substantially binder-free stratum of silver halide microcrystals, which medium incorporates means which, while allowing ready processing of the stratum, will provide protection for the stratum prior to processing and, after processing, protection for an image developed from said stratum.

Other objects of the present invention are to provide a medium of the type described which includes a selected organic polymeric layer over the photosensitive stratum thereof, which layer contributes to protect both said stratum and any silver image afterformed therein; to provide a medium of the type described wherein the photosensitive, microcrystalline, binder-free silver halide stratum is disposed between two selected, solid, organic polymeric layers; and to provide a medium of the type described wherein the two layers (at least in the areas whereadjacent development forms a silver image [following an image-wise exposure), are selected as layers which will become bonded to form a protective coat of said silver;

' yet other objects of the invention are to provide a method medium and a second polymer is incorporated into a processing solution.

Other objects of the present invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the processes involving the several steps and the relation and order of one or more of such steps with respect to each of the others, and the products and compositions possessing the features, properties and the relation of elements which are exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the claims. For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawing wherein there is shown a diagrammatic cross-sectional view of one embodiment of this invention, the relative dimensions of the parts thereof being exaggerated for the purpose of clarity.

The above objects are attained in accordance with the present invention which, in one aspect as shown in the drawing comprises a suitable support 20, such as a sheet material, having a selected subbing layer 22 on one surface thereof. Subbing layer 22, in turn, supports a substautially binder-free, homogenous, microcrystalline, photosensitive, thin silver halide layer or stratum 24. Stratum 24 is preferably sandwiched or disposed between subbing layer 22 and a selected protective layer 26.

The support material may be transparent, translucent, or opaque to radiation actinic to the silver halide. Material suitable for forming support 20 includes papers of all types, such as are used in making photosensitive silver halide emulsion material, in the form of sheets, films, and plates. Other suitable materials are various insoluble (i.e. in treating or processing solutions such as developer solution) and preferably relatively inert synthetic resins and high molecular weight polymers. For instance, where it is intended to use a predetermined developer solution which is aqueous, suitable materials for the support include addition polymers, e.g. polymers of vinyl chloride, vinylidene chloride, vinyl acetate, styrene, isobutylene and acrylonitrile and their copolymers; linear condensation polymers, e.g. polyesters such as polyethylene terephthalate, polyamides such as polyhexamethylene sebacamide, polyester amides such as polyhexamethylene adipamide/ adipate; cellulose derivatives, e.g. cellulose triacetate; vitreous materials such as glass,'ceramic or the like, and others.

The support can be either permeable or impermeable to the developer vehicle as desired. The support is provided with a coating or subbing layer 22 preferably entirely covering one surface. The subbing layer may be employed to serve one or more of a number of ancillary functions, e.g. to alter support opacity, to change support reflectivity, or to modify adherence of microcrystalline silver halide layer 24 to the support, but is here primarily concerned with providing a high molecular Weight, organic polymeric material which, under predetermined conditions, cooperates with the material of layer 26 to form a protective coating about silver reduced from the silver halide microcrystals. Layer 22 is substantially solid, i.e. neither liquid nor viscous, but firm and dry to touch. Where it is intended that exposure of the medium be through the support, layer 22 is preferably transparent; alternatively, if exposure is intended to be made through protective layer 26, subbing layer 22 can be opaque. Similarly, depending on how the medium is intended to be developed in a solution, subbing layer 22 can be either permeable or impermeable to the vehicle of the developing agent.

Layer 26 also comprises an organic polymeric material which is substantially solid (as hereinbefore defined), can be soluble or permeable in or to a selected silver halide developer solution or vehicle, and, of course, can be either substantially transparent or opaque to actinic radiation, all depending on the desired process by which silver halide layer 24 is to be exposed and developed.

A microcrystalline binder-free silver halide layer of the present invention may be formed by vacuum evaporation techniques similarto those described in the aforesaid French patent. A basic machine of the kind useful to form such media by vacuum evaporation is illustrated and described in the book Vacuum Deposition of Thin Films by L. Holland published by John Wiley & Sons, New York City, 1948, pp. 7-8.. Vacuum coating apparatus of this type is well known and usually comprises an evacuable container such as a bell jar, and means for evacuating the latter to an ambient pressure which is preferably less than approximately 1 l mm. of Hg. The latter appears to be the maximum pressure at which reasonably desirable layer formation can be achieved. Included within the bell jar is a crucible or boat which is intended to carry the material which is to be evaporated. Means for heating the boat are usually included, and in one form of the apparatus, the boat is made of tungsten provided with leads to a source of electrical power so that the boat forms a filament which can be heated electrically. By this method the temperature of the evaporating material can be readily controlled. Other methods for heating the starting material may also be used, for instance electrical induction. The apparatus also includes means for holding material such as support sheet 20, at a predetermined location within the bell jar wherein layer 22 on the surface of the support sheet faces the boat, thus insuring that the stream of vapor from the evaporating material in the boat will strike layer 22 and condense thereon to form a film or layer 24 according to the wellknown principles of evaporation techniques. The silver halide layer 24 thus formed is preferably selected to be in the range of about 0.1 to about 0.6 microns in thickness, and should be substantially uniformly thick at whatever value within that range is selected. Preferably, the thickness of the layer is around 0.31:0.2n inasmuch as optimum photographic parameters such as gamma, density, and speed are found within this range.

In one method of using this apparatus to form the medium of the invention, a quantity of silver halide (e.g. of purity 99.99% or better) is placed within the container, for example, at a first location. A suitable support sheet having a layer 22 precoated thereon, is placed within the container at a second location displaced from the first location. The container is pumped down to achieve an operating pressure, and the silver halide is brought to a point above its melting temperature at which evaporation will occur. The resulting vapor is then condensed upon layer 22 to form the microcrystalline silver halide layer.

It is preferred that the temperature of the silver halide melt and the pressure of the system be substantially stable during the silver halide deposition operation; for example, when evaporating silver bromide, the temperature may be kept at about 560 C. and the pressure at about l0 mm, of Hg. A silver-halide deposition process which can be used to form the silver halide stratum of the medium of the invention is described in detail in the aforesaid French patent.

Among the silver halides which are useful in evaporation processes to form the microcry-stalline binder-free silver halide layer of the present invention, are silver bromide, silver chloride and silver iodobromide. The thickness of the silver halide layer formed from these materials may be readily controlled by adjustment of the evaporation temperature, the evaporation time and the distance between the silver halide source and the support sheet, or any combination thereof. The layer thus provided (which is a thin layer preferably within the range described above) appears to be approximately within 4 10% of the density of that of a solid macrocrystal of the same silver halide.

The support sheet 20 with its subbing layer 22 hearing an appropriate stratum 24 of silver halide is then removed from the apparatus, and coated, by known techniques, with protective layer 26.

The polymers useful as materials, out of which layers 22 and 26 can be formed, are those which are film-formers, i.e. materials which can be applied, for example, in liquid form either per se, or from a liquid solution, and will, under normal conditions of temperature and pressure, solidify to form a substantially dry coating which may be transparent to actinic radiation, preferably visible light. For the purposes of the present invention, suitable polymeric materials for layers 22 and 26 are also necessarily those which will cooperate with one another, as a result of the processing of the medium which provides a silver image, to produce a coat by which the image is protected at least from mechanical damage.

Without being restricted to any particular theory, it is believed that the processing of the medium causes the polymers in the layers to cross-link. Where the polymeric materials are soluble in the developer vehicle, cross-linking would tend to insolubilize the polymers. Such a phenomenon apparently does occur. For example, if a photographic medium having the requisite microcrystalline, binder-free silver halide stratum 24, has layer 26 formed thereon from a vinylpyrrolidone/vinyl acetate (PVP/VA) copolymer, and includes either a substrate layer 22 of an incompatible polymer such as cellulose triacetate or no substrate layer at all, in the processing of such medium involving development in a solution, it is found that the PVP/VA dissolves completely in the solution. On the other hand, as detailed hereinafter in Example I for instance, it will be seen that with proper choice of polymers for substrate layer 22, such as a PVP/VA, layer 22 will emerge from the processing incorporated at least in part in an insoluble coating.

Polymeric materials useful to form layers 22 and 26 of the invention include natural polymers, such as gelatin: synthetic polymers, such as polyvinyl pyrrolidone, copolymers of polyvinyl pyrrolidone and vinyl acetate, polyethyleneimine, and others which are soluble or permeable film forming polymers capable of being cross-linked by photographic processing. The polymeric materials may be formed into layers 22 and 26 by any of several methods including coating, casting, and extrusion and subsequent drying. Protective layer 26 thus formed is, for one embodiment of the invention, preferably thin enough to allow a developer solution to dissolve the same or permeate therethrough in a relatively short time (e.g. a matter of seconds or at the most minutes) and is therefore preferably not much thicker than one or two mil inches.

In yet another embodiment of the invention, a silver image can be provided which is embedded or encased in a tough, insoluble polymeric matrix by a process which employs a medium subcoated as hereinbefore described, but completely lacking an overcoat 26. In such event, the material from which the overcoat 26 is normally made is instead dissolved in a solvent compatible with a developer solution and the exposed medium is processed in the mixture of the solutions of polymer and developer.

The media of the invention are employed to form photographic images by exposing the media to an imagewise distribution of actinic radiation or light to form a developable latent image, and then selectively reducing the silver halide at the latent image to form a silver image. The selective reduction is accomplished by developing the medium in a solution containing a reducing agent, for example, an alkaline aqueous solution containing a developing agent and a silver halide solvent. Typically, one can employ an aqueous solution of pmethylaminophenol, sodium sulfite, hydroquinone, sodium carbonate and potassium bromide, available as D- 19 developer from Eastman Kodak Co., Rochester NY.

The development speed can be retarded, if desired, by diluting the D-19 by forming a solution of 70% thereof with 30% of a 5% gelatin aqueous solution. Other formulations may be used containing developing agents such as hydroquinone derivatives, phenylhydrazine, phenylhydroxylarnine, and many others.

A useful developing solution, sometimes known as Mitchells Developer, for use in the process of the present invention is formulated as follows:

Three stock solutions are prepared, preferably using triply-distilled water. These solutions are:

Solution A decomposes with time, Solution B keeps indefinitely, and Solution C should be kept refrigerated to inhibit decomposition. These solutions are used in equal proportions to make the developer. They are mixed by adding 20 ml. quantities of first, Solution B to Solution A and then 20 ml. of Solution C to the mixture of Solutions A and B. In a modification of this developer, to the mixture of A, B and C described above, there is added 3 ml. of a 1% sodium thiosulfate solution.

The formation of the novel photographic medium of the invention and a process for using such medium to provide images embedded in a tough polymeric matrix may be demonstrated by the following illustrative examples in which percentages of components are given by weight unless otherwise indicated:

Example I A first coating solution was prepared as follows:

Polyvinylpyridine grams 10.0 Polyvinyl butyral resin do 1.3 Methylene chloride do 200 Isopropanol do 400 Methylethyl ketone do Surfactant trace The polyvinyl butyral resin was Butvar B90 available from Shawinigan Resins Corp, Massachusetts. The surfactant was Ucon 50 HB 100, a polyalkylene glycol available from Union Carbide Corp, N.J., and was introduced as about 12 drops.

The above solution was coated onto a surface of a cellulose triacetate support sheet and allowed to dry thereon at room temperature to form a subbing layer. The dried coated sheet was placed in vacuum evaporation apparatus of the type hereinbefore described, and a stratum of substantially binder-free, microcrystalline silver bromide was formed on the coated surface, the evaporation being controlled so that the stratum was approximately 0.3 microns in thickness. Both the subbing coat and the silver bromide of this basic medium respectively exhibited good adhesion to the triacetate support. and to one another.

When exposed for A second in a sensitometer, developed in Mitchells Developer, fixed in a fixative solution of sodium thiosulfate and washed, a silver image formed which adhered well to the support but which could easily be scratched or dusted off with gentle abrasion with ones fingers.

A second coating solution Was prepared from 20 grams of vinyl pyrrolidone-vinyl acetate (PVP/VA) copolymer in 70/30 ratio in 50% ethyl alcohol (available as E735 resin from Antara Chemicals, a division of General Ani- 6 line and Film Corp.) diluted with grams of ethyl alcohol.

Another basic medium comprising a silver-bromide micro-crystalline stratum on a subbing layer on triacetate support, prepared as hereinbefore described in this example, was dipped into the second coating solution to provide a thin (approximately 2 or 3 mil inches) protective layer over the silver halide stratum. After allowing the excess solution to drip off, the coated medium of the present invention was dried at room temperature overnight. This latter medium was then exposed, developed, fixed and washed in the same manner as hereinbefore described in this example. Upon drying, it was found that the silver image formedwas encased or embedded in a tough, insoluble polymeric matrix and could not be scratched off even with a strong fingernail pressure. No adverse effect of the insolubilized coating on the image resolution was observed.

Example II A solution of poly-N-methylethyleneimine (40% solids in H O) was coated to form a subbing layer on a support sheet of polyethylene terephthalate. After being dried, the sheet had vacuum evaporated onto the subbing layer a stratum (approximately 0.3 microns thick) of microcrystalline, binder-free photosensitive silver bromide.

The basic mediumthus prepared was further coated over the silver bromide stratum with the second coating solution as described in Example I, and allowed to dry. Upon an imagewise exposure to form a developable latent image, development in Mitchells Developer, fixing, washing and drying, it was found that the silver image pro duced from the latent image was encased in a tough, insolubilized polymeric matrix which was highly abrasionresistant.

Example III A photographic medium was formed of polyethylene terephthalate support sheeting having a binder-free, microcrystalline,photosensitive silver-bromide stratum of approximately 0.3 microns evaporated ontoa subbing layer produced from the first coating solution of Example I. This medium then had overcoated onto the silver bromide a thin discrete layer or film formed by drying from a second coating solution formulated as follows:

Gelatin (photographic grade) gr 6 Distilled water cc 194 Chrome alum solution (2.5%) cc 6 The completed medium, upon exposure, development in the Mitchells Developer modified by the addition of sodium thiosulfate as hereinbefore explained, and washing, provided an insolubilized polymeric matrix in which the developed silver was firmly embedded.

The same procedure was followed using however, as a support sheet and subbing layer such commercially available photographic base materials as baryta paper, subbed cellulose triacetate and subbed polyethylene terephthalate c-ronar), all of which have commercially prepared subbing layers containing gelatin. In each case, the overcoat was found to have cooperated with the subbing layer and the silver to form a matrix of insolubilized polymer containing the developed silver image and bound to the support.

Example IV Sections of commercially subbed cellulose triacetate with silver-bromide stratum overcoated with gelatin made according to the prior example, were treated as follows:

1) An unexposed section was developed in Modified Mitchells developer-the resulting gelatin overcoat easily peeled off;

(2) A section was exposed and developed in Mitchells developerthe resulting gelatin overcoat also was readily stripped;

(3) A piece was exposed and fixed in an acid fixer again, the gelatin overcoat could be easily stripped;

--(4) -A piece was exposed, developed in Mitchells developer, and fixed in an acid fixerhere, the resulting gelatin was hard and did not strip;

(5) A piece was exposed and developed in Modified Mitchells developer (which, as hereinbefore explained, includes a fixer)again, the gelatin was tough and nonpeel-able.

With relation to gelatin, it is apparent that the insolubilization occurs only when the entire process of exposure, development and fixing occurs.

Example V A basic photographic medium was prepared as described in Example I whereby a silver bromide stratum was evaporated into a cellulose triacetate sheet precoated with a subbing layer prepared from the polyvinylpyridinepolyvinyl butyral solution.

This basic medium was then overcoated with an aqueous solution of poly- N-methylethyleneimine similar to that employed in Example 11.

Upon appropriate exposure, development, fixing, washing and drying as previously described in the previous examples, the resulting silver'image was found to be embedded in a tough, insoluble polymer matrix.

Example VI A basic photographic medium was prepared as described in Example V, but was not overcoated.

This medium was exposed for second in a sensitometer, and then developed in Mitchells developer, modified by the addition of sodium thiosulfate as previously described, and further having added thereto about 1 to of a solution of vinyl pyrrolidone-vinylacetate copolymer in 70/30 ratio in 75% ethyl alcohol. The resulting developed medium was washed in water and dried at room temperature. The silver image thus formed was found to be embedded in a tough, insoluble polymer matrix.

Since certain changes may be made in the above processes and products without departing'from the scope of the invention herein involved it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted in an illustr-ative and not in a limiting sense.

What is claimed is:

1. A photographic medium comprising a support sheet, a first high molecular weight organic polymer disposed in a subbing layer on a surface of said sheet, a stratum of substantially binder-free microcrystalline photosensitive silver halide disposed on said subbing layer, a second high molecular weight organic polymer disposed on said stratum, said polymers being characterized in that upon exposure, development and chemical fixing of said medium said polymers combine in the area of silver formation to form an insolubilized polymeric matrix in which developed silver is embedded, said polymers being different and each being selected from the group consisting of poly-N-methylethyleneimine, a copolymer of vinylpyrrolidone and vinylacetate, and polyvinylbutyral and polyvinylpyridine.

2. A photographic medium as defined in claim 1, wherein one of said polymers comprises po-lyvinylbutyral and polyvinylpyridine, and the other of said polymers comprises poly-N-methylethyleneimine.

3. A photographic medium as defined in claim 1 wherein one of said polymers comprises poly-N-methylethyleneimine, and the other of said polymer comprises a copolymer of vinylpyrrolidone vinylacetate.

4. A photographic medium as defined in claim 1 wherein one of said polymers comprises polyvinylbutyral and polyvinylpyridine, and the other of said polymers comprises vinyl pyrrolidone-vinyl acetate copolymer.

References Cited UNITED STATES PATENTS 3,219,444 11/1965 Lu V-alle et al. 96-94 3,219,448 11/1965 Lu Valle et al. 9694 3,219,451 11/1965 Lu Valle et al. 9694 3,279,920 10/1966 Theodorou 96-94 NORTON G. TORCHIN, Primary Examiner.

C. E. DAVIS, Assistant Examiner. 

1. A PHOTOGRAPHIC MEDIUM COMPRISING A SUPPORT SHEET, A FIRST HIGH MOLECULAR WEIGHT ORGANIC POLYMER DISPOSED IN A SUBBING LAYER ON A SURFACE OF SAID SHEET, A STRATUM OF SUBSTANTIALLY BINDER-FREE MICROCRYSTALLINE PHOTOSENSITIVE SILVER HALIDE DISPOSED ON SAID SUBBING LAYER, A SECOND HIGH MOLECULAR WEIGHT ORGANIC POLYMER DISPOSED ON SAID STRATUM, SAID POLYMERS BEING CHARACTERIZED IN THAT UPON EXPOSURE, DEVELOPMENT AND CHEMICAL FIXING OF SAID MEDIUM SAID POLYMERS COMBINE IN THE AREA OF SILVER FORMATION TO FORM AN INSOLUBILIZED POLYMERIC MATRIX IN WHICH DEVELOPED SILVER IS EMBEDDED, SAID POLYMERS BEING DIFFERENT AND EACH BEING SELECTED FROM THE GROUP CONSISTING OF POLY-N-METHYLETHYLENEIMINE, A COPOLYMER OF VINYLPYRROLIDONE AND VINYLACETATE, AND POLYVINYLBUTYRAL AND POLYVINYLPYRIDINE. 